Little brother=yandere siscon who gets overly jealous, doesn't listen to his sister, no comprehension skill, aka brawl first talk later, a violent character,(because he's the demon lord? even so he was born as a human), not smart enough to know his servants was running amok, and lastly, you don't have any right to approve of whom she befriends with (I kinda understand when you worried for someone, if ever that you don't like that person or is a bad influence to your sister, just talk no need to be violent, the only you can do as a brother is to respect her decision ........, and his reaction is too much.) (I hope he will have a character development)
cat=true to his name (manipulative demon)
elder sister= I kinda understand why she's not sure whether the prince is the cat or actually the real prince because they have the same outfit but the way the prince speaks is kinda obvious. (I guess it's intentional in order to progress the story but I hope she will have also a character development.)
second prince=take advantage of the {misunderstanding} situation (so it's his fault also for why he is in that kind of situation.)